Turbo-encoded, multiple-input multiple-output (“MIMO”) systems have been proposed for the support of high-speed downlink packet access (“HSDPA”) in UMTS. The concept here may be to increase the achievable data rates for a particular user through a combination of code re-use across transmit antennas and higher-order modulation schemes. The code re-use inevitably may result in high levels of interference at the mobile receiver, even under non-dispersive channel conditions. In order to tackle such high interference levels, receivers based on an optimal a posteriori probability (“APP”) detector. To cope with dispersive channels and in order to avoid sequence estimation, it may be necessary to use an APP detector preceded by a matrix channel equalizer.
Essentially, the APP detector may operate by computing soft-outputs for the transmitted bits, which most closely match the received signal in an Euclidian sense. The computational complexity of the APP detector may be an exponential function of the total number of bits transmitted during a symbol epoch, which is equal to the product of the number of transmit antennas and the number of bits per symbol. Consequently, the complexity of the APP detector can become prohibitive for increasing numbers of transmit antennas and modulation orders. This inflexibility of the optimal APP detector has resulted in renewed interest in the use of suboptimal but less complex MIMO detectors.
Successive interference cancellation (“SIC”) schemes have been considered for many years in the context of multi-user detection for the CDMA uplink. These schemes may combat interference by successively detecting and canceling the influence of data streams from the received signal. The more reliable data streams may be detected and cancelled first. In the context of MIMO receivers, a SIC architecture incorporating ordering and detection may be based on the minimum mean-squared error (“MMSE”) criterion. Furthermore, significant performance improvements might have been demonstrated through iterations between the MIMO detector and a convolutional decoder.